1. Field of the Invention
The present invention relates to a color converting circuit for use in an image forming apparatus such as an electrophotographic copying machine.
2. Description of the Prior Art
Conventionally, some image forming apparatuses such as digital electrophotographic copying machines have been provided with a color converting function to copy a color original while converting a color on the original, which color is specified by the user (this color will hereinafter be referred to as color-to-be-converted), to another color desired by the user (this color will hereinafter be referred to as conversion color). Specifically, in executing predetermined signal processing at an image processing circuit portion with respect to each of yellow, magenta and cyan color image data which are read out by a color scanner from a color original to be copied, color image data of a color-to-be-converted which are among color image data to be inputted, are respectively converted to color image data of a conversion color at a color converting circuit provided in the image processing circuit portion, and thereafter, the converted data are outputted to a succeeding laser scanner unit for forming a latent image. A color image where color conversion has been made is thus formed.
In converting color image data at the color converting circuit in the above-described manner, it has been considered to perform the conversion with a predetermined latitude (i.e. conversion range) so that nonuniformity of each color image data of a color-to-be-converted due to nonuniformity of the reading out by the color scanner and color nonuniformity on the original can be coped with, that is, so that a specified color conversion processing is executed without failure.
With such a conversion method, however, when a color of an extremely low density, that is, a color of a density close to that of the color of the background portion of the original where no letters, figures, etc. are printed (this color of the background portion will hereinafter be referred to as paper color) is specified from among the colors on the original, due to a certain conversion range, the paper color of the original may be erroneously converted together with the specified color. For example, when pale pink having a density close to that of the paper color (generally, white or pale grey) of the original is specified as the color-to-be-converted, depending on the setting of the conversion range, the paper color of the original may be converted together with pink.
Moreover, in the above-described conventional color converting circuit, since a conversion range is not set which is in accordance with a density characteristic of each of the color image data to be input, depending on whether the color-to-be-converted is a color of low density or a color of high density, erroneous conversion may be caused or some portions may not be converted at all.
Further, in the conventional color converting circuit, since a conversion range is not set which is in accordance with an image quality mode (in this case, copy mode) of the copying machine which mode the user arbitrary selects according to the original, depending on whether the image quality mode is a letter mode or a photograph mode, erroneous conversion may be caused or some portions may not be converted at all.
Specifically, in the case of the letter mode, since it is a copy mode for more excellently reproducing a multi-color original (e.g. a color graph using primary colors) for use in offices which original has only a little half tone and, in the case of monochrome originals, an original having a sharp contrast, that is, since it is a mode for copying an original having no half-tone-level density, no problems arise even if the conversion range is set to be relatively large so that no unconverted portions are produced. In the case of the photograph mode, however, since it is a copy mode for more excellently reproducing an original (e.g. photograph) having a large amount of half tone, if the conversion range is set to be too large, a color other than the specified color may be erroneously converted.